FrameSet: Fix inheritance of 'border' attribute

* We should access border_ only if border_set_.
* We should refer to the parent's Border() otherwise.

Bug: 1346221
Change-Id: Ibe1f279a6dba5c93e6ab9e4d9b15e2ffbe2c3c8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3858736


Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040242}